1. Field of the Invention
The present invention relates to a NAND flash memory.
2. Related Art
In a flash memory, writing and erasing of information, that is, injection and removal of charges, are performed by a tunnel current in a high electric field from a silicon substrate or a control gate, or by a hot-carrier from a silicon substrate.
When writing and erasing of information are repeated, a charge passes through a gate dielectric film repetitively, regardless of any method of injection. As a result, the gate dielectric film is damaged, and many electron traps and hole traps are generated within the gate dielectric film. The charge traps expand a distribution width of a threshold voltage of memory cells.